Hello everyone! I’m having a bit of a problem. I have a 2007 C6 (auto) and I’m having all sorts of lights popping up on my dash. I’ve seen other posts about the same issue but I’ve never really seen an exact answer to them. The car will no-crank, almost all the gauges (not including rpm) will hit 0, abs light, check engine, service traction, low fuel, etc. and the transmissions stays on 2nd gear. I hooked up my OBD and the codes were U0073(Control Module Communication Bus “A” Off), U0100(Lost Communication to ECM/PCM “A”), & P0700(Transmission Control System [Mil Request]). I took it to the shop last week and they told me that the harness connecting to the transmission was covered in fluid. So, they changed out the value body, cleaned the harness, & changed the TCM. Once I got it back everything was running fine until today when I tried to WOT and all the same symptoms appeared. I’m also having tire pressure problems. As in I have the proper tire pressure but it will register my Rear Left wheel as under pressure as well as my Front Right wheel above 50 or sometimes “XX” when clearly all tires have the correct tire pressure. Some of my friends believe there is a shortage somewhere in the vehicle. What do you guys think? Any input would be appreciated!

 

Check this TSB:

#08-07-30-021B: Loss of High Speed GMLAN Communications, Intermittent No Crank, IPC Gauge Fluctuation, Intermittent Door Lock Cycling, Transmission May Not Shift, DTCs U0073, U0100, U0101, U0102, U0121, U0140 Set - (Sep 8, 2008)


Subject: Loss Of High Speed GMLAN Communications, Intermittent No Crank, IPC Gauge Fluctuation, Intermittent Door Lock Cycling, Transmission May Not Shift, DTCs U0073, U0100, U0101, U0102, U0121, U0140 Set (Repair Backed Out Terminal In Transmission Harness Connector)


Models: 2009 Cadillac Escalade Hybrid

2007-2009 Cadillac Escalade Models, XLR

2007-2009 Chevrolet Corvette, Silverado, Suburban, Tahoe

2008-2009 Chevrolet Tahoe Hybrid

2008-2009 GMC Yukon Hybrid

2007-2009 GMC Sierra, Yukon Models

with 6 Speed 6L80 (RPO MYC), 6L90 (RPO MYD) or Hybrid Two Mode 2M70 Automatic Transmission


--------------------------------------------------------------------------------

This bulletin is being revised to add the 2009 model year and the 2009 Cadillac Escalade Hybrid. Please discard Corporate Bulletin Number 08-07-30-021A (Section 08 -- Body & Accessories).


--------------------------------------------------------------------------------

Condition
Some customers may comment on any of the following conditions:

• The transmission may not shift.

• The door locks may cycle by themselves.

• The engine may not crank intermittently.

• The Instrument Panel Cluster (IPC) gauges may fluctuate.

Technicians may find the following High Speed GMLAN Communications Diagnostic Trouble Codes (DTCs) shown as a current code or stored in history:

• U0073: Control Module Communications Bus OFF

• U0100: Lost Communication With ECM/PCM

• U0101: Lost Communication With TCM

• U0102: Lost Communication With Transfer Case Control Module

• U0121: Lost Communication With Anti-Lock Brake System (ABS) Control Module

• U0140: Lost Communication With Body Control Module

Cause
These conditions may be caused by a terminal for the High Speed GMLAN Serial Data Bus that has backed out of the 16-way electrical connector to the automatic transmission.

 

we have done this already. When checking the 16 terminalplug to the transmission, it was covered with fluid. The mechanic then cleaned and plugged back the plug and changed the valve body & the TCM. Still having the same problem. I’ve also noticed today while it was acting up that when I would go onto a bumpy road, the mph guage would flicker as if the bumps would flicker the mph gauge to work.

 

I wasn’t able to get it on video, but usually the mph gauge will start flickering then it hit 0mph. Also, the traction control light would be on with the rest but it wasn’t on in this video but is usually the first 2 lights (active handling & traction control) to turn on after the flickering of the mph gauge.

 

Update: I took it back to the shop after they cleaned the harness, replaced the valve body & TCM. They ended up replacing the end of the harness that plugs into the transmission & told me that the module behind the right wheel wall was soaked with water (not the BCM behind the footwall). After having it for 2 weeks its started acting up again.

I’m taking it back to the shop but I feel like these mechanics will never find the problem to it and just have me spending my money like crazy. Do you guys believe the problem to this could be the rubber drain pipe on the passenger side under the car battery could be plugged and overflowing onto the module?

 

Update: I took it back to the dealership & they seemed puzzled when I told them it was acting up again. Hard to get out of park, cruise control won’t work while driving, AC won’t blow cold air when these codes happen, car goes into limp mode, MPH gauge flickers, the rest except the RPM guage & Battery guage, & abs light, check engine, service traction, low fuel, etc. They ended up changing a power connector to the main computer (which I’m assuming is the BCM.) They also told me if this doesn’t work then theyre basically done because they can’t find the problem.

The car was running great up until the 3rd week. Same problems occured. I hooked up my obd2 scanner & it reports the same codes u0073 & u0100.

I also did an ABS scan & the following codes popped up: C0161-12 Antilock Braking System Brake Switch Circuit - Low Input, U2100-00 1. Controller Area Network (CAN) 2. CAN Bus Communications. Upon further review these codes can affect cruise control & can have problem with getting out of park, which I had before.

When I first got the vehicle from the used dealership, they replaced the transmission with a used low milage transmission & when these codes began to pop up I believe they switched out the ABS Brake switch or something like that. I can’t remember & the owner dealership wasn’t clear on his explanation either.

Can anyone help me with this problem? I already been a whole year since I’ve had this problem & still can’t find any solutions. Any help would be appreciated. I’ve been daily driving like this for a whole year & it’s extremely frustrating. Could it be a bad ECM/PCM causing all this havoc & DIC codes?

Just to further clarify, these are the parts that were changed or “fixed”: ABS Brake switch at the brake, low milage automatic transmission, Valve Body, new TCM, the pigtail end of the 16 pin connector of the gmlan to the side of the transmission, & a power connector to the BCM.

 

Check Grounds. I have a few of these error coded but my main one is P1682 (from what I am learning and understand, this one error code is triggering others). C5 Diag (a member here) checked almost every wire in my car and he thinks the problem is in the fuse box. He has a post up in Tech and will update it.

 

I just checked all the grounds & they were good. I removed the battery & checked the rain duct to see if it was clogged up and overspilled on the ECM/PCM & it was clean as a whistle. I also checked every fuse in the fuse box & they were fine as well. I even removed the fuse box (by unscrewing the 4 bolts inside the fuse box) & didn’t see any pins with corrosion or any of that sort.

After checking everything, I did find a light blue wire somewhat damaged inside a harness. I’m not good with wiring but I’m assuming it’s connected to the ECM/PCM below? I’m not quite sure what that wire is for.

 

Been following thread, have a hunch damaged wire(s) have something to do w/ this matter.
Frankly codes are pretty weird, rarely reported and almost always one-off problems.
Just locating one of these flukes can drive one nuts, break 'em or both.

No idea what that bundle is or where it goes, but its a good place to start.
Be awfully nice if bundle can be opened for close inspection.
If there's one or more broken/chaffed wires, offenders can carefully be spliced back together.
Only alternative is sourcing that bundle (harness?), buying/installing.
PITA yet if it fixed issue, be worth it.

Knowing C6 that *thing*, bundle, harness probably can't be cheap, those needing it have to have one & $ goes up.
Repairing's doable though, IF there's a real issue w/ it.
BTTT

 

Sorry for the month long delay, I’ve been super annoyed with the car. I ended up taking it back to the same shop for two weeks. Within those two weeks, all they told me is that they believe it has to do with the transmission. I ended taking the car out because they barely were doing anything with it because they were “overcrowded with cars to work on”.

New issues have came up as well. Every time I back out of my driveway and go from R to D, I wait for the gear to engage & once I feel the car jolt forward to engage the gear, “Service Traction Control” & “Service Active Handling” lights come on at the same time I put it into Drive.

At this point I’m just thinking about trading it in for a new truck.

 

If you trade it in, it sounds like that one is destined for an auction for sure!

 

It sounds like something is stomping on your car's high-speed CAN bus (a.k.a. "GMLAN").

The tech needs to verify bus termination, and see if he can isolate the wiring fault or misbehaving module. There are only five or six modules on the bus in a C6. It may be helpful to view the signal on a scope.

Here are two easy things you can try yourself:

1) with the car off, check the resistance between terminal 6 and terminal 14 of the DLC (the "OBD2 port" under the dash). It should be 60Ω.

2) Unplug the big blue connectors at the BCM in the passenger footwell and drench both sides in electrical contact cleaner. The terminals have a tendency to get dirty, and the CAN bus lines are among them.

 

Hi ssebastian65,

Have you noticed that the problem happens more after you wash the car? I had a similar problem where the large harness connector in the right front fender next to the battery would get wet. The connector had a bad seal and water got into the connector. I think when I hit a bump some of the moisture would cause a brief short and put the car in limp mode.

I had a long thread on the topic and also posted a diagram of how the modules connect to the High Speed network that might help you track down the problem.

Good luck,
Frank

 

Did you ever find the problem? My 09 c6 is having the Same problem.
Thanks

 

Hi brex71,

I did find the problem and resolve it (fingers crossed, good for the last two years). Here are a bunch of post on my issue. Feel free to private message me to compare notes. Basically, the x184 connector had a bad seal allowing water into the connector and causing a short on the CAN bus.

Original post with video of the dashboard:
https://www.corvetteforum.com/forums...-light-on.html

CAN bus outline and additional material:
https://www.corvetteforum.com/forums...or-the-c6.html

Pictures of the connector:
https://www.corvetteforum.com/forums...sues-x184.html

Good luck,
Frank

 

I found pushing those big blue connectors by the passenger seat footwell remedies the problem I think my subwoofer rattles them lose than all the lights come on hope this helps